(a) Field of the Invention
The present invention relates to a wide-angle retrofocus type of objective lens system for endoscopes.
(b) Description of the Prior Art
In clinical inspection with an endoscope, it is necessary to observe or photograph a wide range of inspection area at a time so as to locate abnormal portion within the sight. For this reason, objective lens systems having wide angles are desired for endoscopes. However, the conventional objective lens systems for endoscopes had field angles of about 80.degree. at widest. Therefore, the endoscopes using these objective lens systems permitted observing or photographing narrow ranges only at a time, required long time for clinical inspections, made patients suffer from much pain or large burden, and had rather high possibility to make abnormal portions out of sight.
In order to widen field angle of an objective lens system for endoscopes, it is generally known to design the so-called retrofocus type of lens system in which a diverging lens group and a converging lens group are arranged as the front and rear lens groups respectively. This refrofocus type of lens system has excellent features to permit locating the rear focal points apart from the final lens surface, avoiding influences due to contamination and injury on lens surface and correcting curvature of field over a wide field angle owing to a concave lens component having strong power in the front lens group.
However, when the retrofocus type of lens system having the above-described excellent features is so designed as to have such a ultra-wide angle as is mentioned as the object of the present invention, it has drawbacks. Speaking concretely of the drawbacks, the lens system allows spherical aberration and lateral chromatic aberration to be produced by the negative refractive power which is too strong in the front lens group.